1. Field of the Invention
This invention relates generally to a system and method for identifying leaks in a cathode sub-system of a fuel cell system and, more particularly, to a system and method for identifying air leaks in a cathode sub-system of a fuel cell system that includes monitoring the air flow into a compressor when valves are positioned so that air flows only around the compressor.
2. Discussion of the Related Art
Hydrogen is a very attractive fuel because it is clean and can be used to efficiently produce electricity in a fuel cell. A hydrogen fuel cell is an electro-chemical device that includes an anode and a cathode with an electrolyte therebetween. The anode receives hydrogen gas and the cathode receives oxygen or air. The hydrogen gas is dissociated in the anode to generate free protons and electrons. The protons pass through the electrolyte to the cathode. The protons react with the oxygen and the electrons in the cathode to generate water. The electrons from the anode cannot pass through the electrolyte, and thus are directed through a load to perform work before being sent to the cathode.
Proton exchange membrane fuel cells (PEMFC) are a popular fuel cell for vehicles. The PEMFC generally includes a solid polymer electrolyte proton conducting membrane, such as a perfluorosulfonic acid membrane. The anode and cathode typically include finely divided catalytic particles, usually platinum (Pt), supported on carbon particles and mixed with an ionomer. The catalytic mixture is deposited on opposing sides of the membrane. The combination of the anode catalytic mixture, the cathode catalytic mixture and the membrane define a membrane electrode assembly (MEA). MEAs are relatively expensive to manufacture and require certain conditions for effective operation.
Several fuel cells are typically combined in a fuel cell stack to generate the desired power. For example, a typical fuel cell stack for a vehicle may have two hundred or more stacked fuel cells. The fuel cell stack receives a cathode input reactant gas, typically a flow of air forced through the stack by a compressor. Not all of the oxygen is consumed by the stack and some of the air is output as a cathode exhaust gas that may include water as a stack by-product. The fuel cell stack also receives an anode hydrogen reactant gas that flows into the anode side of the stack. The stack also includes flow channels through which a cooling fluid flows.
The fuel cell stack includes a series of bipolar plates positioned between the several MEAs in the stack, where the bipolar plates and the MEAs are positioned between the two end plates. The bipolar plates include an anode side and a cathode side for adjacent fuel cells in the stack. Anode gas flow channels are provided on the anode side of the bipolar plates that allow the anode reactant gas to flow to the respective MEA. Cathode gas flow channels are provided on the cathode side of the bipolar plates that allow the cathode reactant gas to flow to the respective MEA. One end plate includes anode gas flow channels, and the other end plate includes cathode gas flow channels. The bipolar plates and end plates are made of a conductive material, such as stainless steel or a conductive composite. The end plates conduct the electricity generated by the fuel cells out of the stack. The bipolar plates also include flow channels through which a cooling fluid flows.
There are many components, devices and elements in a fuel cell system through which the reactant gases flow both upstream and downstream of the fuel cell stack. For example, in the cathode sub-system, the compressor provides air flow to the cathode side of the stack typically through a charge air cooler that cools the compressed air heated as a result of the compression and a water vapor transfer (WVT) unit that humidifies the cooled air, generally using the cathode exhaust, before the air is sent to the stack. The cathode sub-system also typically includes a by-pass valve for by-passing air around the stack and a back-pressure valve in the cathode exhaust line that controls the cathode side pressure. Any of these devices and components can develop leaks over time where air may be dumped overboard before it reaches the fuel cell stack, which reduces the amount of reactant air provided to the fuel cell stack, thus causing performance issues. In other words, the control algorithms for the fuel cell stack may command the compressor to a certain speed for a desired stack output current, but that amount of air may not reach the stack because air leaks occur through one or more of the components before the stack. Therefore, it would be desirable to be able to determine that a significant leak is occurring in the cathode sub-system as a diagnostic tool.